bug#38011: 27.0.50; [PATCH] WIP on allowing Gnus backends to return header data directly

[Eric Abrahamsen <eric@ericabrahamsen.net>]

Lars Ingebrigtsen <larsi@gnus.org> writes:

> Eric Abrahamsen <eric@ericabrahamsen.net> writes:
>
>> Can someone explain what exactly this function is supposed to do?
>
> Somebody should have written more comments when they wrote that...  and,
> like, made it correct.
>
> It transforms this:
>
> 64007 Formatting labels used for URL buttons in Gnus articles Narendra
> Joshi <narendraj9@gmail.com> Sun, 19 Apr 2020 00:31:13 +0200
> <mailman.633.1587249088.3066.help-gnu-emacs@gnu.org>
> <87eesk2zpq.fsf@gmail.com> 3614 14 Xref:
> reader01.eternal-september.org gnu.emacs.help:57603
>
> Into this:
>
> 64007 Formatting labels used for URL buttons in Gnus articles Narendra
> Joshi <narendraj9@gmail.com> Sun, 19 Apr 2020 00:31:13 +0200
> <mailman.633.1587249088.3066.help-gnu-emacs@gnu.org>
> <87eesk2zpq.fsf@gmail.com> 3614 14 Xref: marnie gnu.emacs.help:57603
> 01.eternal-september.org
>
> Which is wrong, of course -- the "01.eternal-september.org" thing
> shouldn't be there.
>
> Anyway, what it's supposed to do it rewrite
>
> Xref: reader01.eternal-september.org foo.bar:2523 gnu.emacs.help:57603 zot.bar:3242
>
> to
>
> Xref: whatever gnu.emacs.help:57603 foo.bar:2523 zot.bar:3242
>
> That is, put the group/article we're really selecting first in the Xref
> header, but leave the remaining as they were.  This is because we need
> those to mark the article as read in those other groups, but we
> primarily need to know where this article really came from (the first
> entry).

Slowly, slowly, I'm getting this done. I'm still a bit confused here,
though. The xref elements look like they're not supposed to have spaces
in them, but the existing code does this:

(insert "Xref: " sysname " " group ":")
(princ article (current-buffer))

Which leaves a space between sysname and group.

You say the existing xrefs should be left as they are, but the code adds
"prefix" to them. Should this be added unconditionally?

Here's the new version of the function, operating on a header struct.
Does this look right to you?

Thanks,
Eric

(defun nnvirtual-update-xref-header (header group prefix sysname)
  "Add xref to component GROUP to HEADER.
Also add a server PREFIX any existing xref lines."
  (let ((bits (split-string (mail-header-xref header)
			    nil t "[[:blank:]]"))
	(art-no (mail-header-number header)))
    (setq bits
	  (mapcar (lambda (bit)
		    (concat prefix bit))
		  bits))
    (setf (mail-header-xref header)
	  (mapconcat #'identity
		     (cons (format "%s %s:%d"
                                   sysname group art-no)
			   bits)
		     " "))))